Celestial bodies dictate the calendar and once stopped a war. John Barrow ponders their effect on the human race

The Christmas story follows a star - or at least a triple conjunction of Saturn and Jupiter in Pisces. Yet, surrounded by the nocturnal glare of artificial light that emanates from our cities, we see little of the stars. They have no significance at all for most of us. For the ancients, especially those living below clear skies or in the rarefied air of mountainous regions, things were very different. The spectacle of the star-spangled sky would have been the most impressive sight of their lives. No wonder that stories grew up in which these pieces of celestial jewellery played a starring role.

At the beginning of this century, the philosopher George Santayana picked out the appearance of the night sky as an exemplar of what is attractive to the human mind: a level of intricacy delicately poised between bewildering complexity and banal simplicity. A hint of intriguing pattern challenges the mind to ponder and seek it out.

If we were to see the night sky for the first time, imagine the consequences of such an astral awakening. It was just such a contemplation that inspired the young Isaac Asimov to pen his famous short story Nightfall about the final days of the civilisation on the planet Saro. This strange world basked in the light of six suns. Natural darkness was unknown; so, therefore, were the stars. The inhabitants evolved in a world of light with a strong fear of the dark, and an understandable susceptibility to claustrophobia. Their astronomers were convinced of the smallness of the universe. Unable to see beyond their own sixfold solar system, they contented themselves with showing how its complicated motions could be understood using the same law of gravitation that worked so well on the surface of Saro.

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These rationalists shared their world with romantic cultists, who perpetuated an "old knowledge" of a world of stars beyond the sky and a coming of darkness when the world would end.

Social tensions in Saro mount when the astronomers discover that there must be an unseen moon in their solar system that will only become visible when it eclipses one of the suns. A few astronomers realise that the moon will eclipse the second sun of the system, at a time when it is the only sun in the sky. The eclipse will be total. News of this expectation leaks out. Civil unrest mounts as the cultists stir up eschatological fever. Darkness blots out the sky and tens of thousands of brilliant stars appear, shrouding the planet in a canopy of twinkling starlight. For Saro is not a denizen of the sparsely-populated stellar suburbs of a galaxy like the Milky Way; it lies deep in the dense heart of a star cluster. Panic and civil unrest breaks out. There, the story ends; the reader is left to ponder the revolution in outlook that is about to occur.

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The simple question of why the sky is dark at night is surprisingly subtle and nothing to do with the sun. It was first raised by Edmund Halley (he of comet fame) in 1721, before being resurrected by Wilhelm Olbers, a German astronomer, in 1823. Nowadays, it is known as Olbers' paradox. A paradox because it appeared that a universe filled with stars should result in our line of sight ending on a star in whatever direction we look. When we look into a dense forest our line of sight always ends on a tree, so when we gaze out into the universe why does the whole night sky not look like the surface of an overlapping forest of stars?

The simple answer to Olbers' paradox seems to be that there is too little matter in our universe to produce enough energy to make the entire sky bright. If you could snap your fingers and convert all the material in the universe into radiation it would merely raise the temperature of the ambient cosmic background radiation from three to 30 degrees above absolute zero. We would not notice anything amiss. But the reason the universe contains such a low density of matter is because it has been expanding and rarefying for more than ten billion years.

This vast expanse of time is not an example of cosmic procrastination. It is needed for stars to form and then produce the elements heavier than helium that are required for the spontaneous evolution of any form of chemical complexity. The dark night sky is a by-product of those conditions that are required if a universe is to be habitable by living things. Once, billions of years ago, when the universe first emerged from the Big Bang, the sky was bright - brighter than a billion suns - but no one was there to see it.

The influence of the night sky upon our own civilisation has not been quite so cataclysmic as it was on the fictional world of Saro, but it has nevertheless been deep and far reaching. By coincidence the respective distances from us of the sun and the moon conspire to present the same apparent sizes on the sky, despite the huge difference in their true sizes. As a result we see complete eclipses of the sun.

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Ancient eclipses are famous for their influence upon human affairs. The total eclipse that occurred on May 28 in 585bc was so dramatic that it ended the five-year-old war between the Lydians and the Medes. Their records tell us that in the midst of battle "the day was turned into night"; at once their fighting stopped and a peace treaty was signed. In contrast, the eclipse of the moon on August 413bc brought about a less amicable end to the Peloponnesian war between the Athenians and the Syracusians. The Athenian soldiers were so terrified by the eclipse that they became reluctant to leave Syracuse, as planned. Interpreting the eclipse as a bad omen, their commanders delayed the departure for a month. This delay delivered all their forces into the hands of the Syracusians - the procrastinating commander was put to death.

In 1503 Christopher Columbus exploited his knowledge of an eclipse of the moon by the sun to enlist the help of the Jamaicans after his damaged ships were stranded near their island. At first, he traded trinkets with the natives in return for food, but after a while they refused to provide any more provisions. Columbus's response was to arrange a conference with the natives on the night of February 29, 1504 - the time when an eclipse of the moon would begin. He announced that his God was displeased by their lack of assistance and was going to remove the moon as a sign of his deep displeasure. As the Earth's shadow began to fall across the moon's face, the natives quickly agreed to provide him with anything he wanted, so long as he brought back the moon. Columbus informed them that he would need to withdraw and persuade his God to restore the lesser light to the heavens. After retiring with his hourglass for an appropriate period, he then returned in the nick of time, to announce the Almighty's pardon for their sins and the restoration of the moon to the sky. Soon afterwards, the eclipse ended. Columbus had no further problems on Jamaica; he and his men were subsequently rescued, and returned in triumph to Spain.

Thus, there is more to the sky than meets the eye. Our timekeeping is a curious mixture of astronomy and astrology. The days are dictated by the period of the spinning earth, the months by the moon's motion, the year by our long slow orbit of the sun, and the seasons by the inclination of the Earth's polar axis to the plane of that orbit, and the week by the number of large bodies periodically wandering through the heavens in ancient times. Our uncertain sense of place in the universe and our feelings for the transcendental owe much to the vastness and darkness of space around us. Our mythologies and legends are infused with personifications of the sky's patterns, our science rests upon the insights we gained by the precise measurement of its clockwork. Most striking of all, the rays of sunlight that fall upon the Earth's atmosphere create the clear blue sky, the red of sunset, and clouds of white that fill the world with colour. Sunlight makes colour vision advantageous and periodically promote so much of that extravaganza of diversity that we call life. But when the darkness comes the whole world changes again.

John D. Barrow is professor of astronomy at the University of Sussex. His latest book The Artful Universe has just been published by Oxford University Press.

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